Anticorrosive treatment of zinc and metallic materials coated with zinc



United States Patent Oifice 3,437,574 ANTICORROSIVE TREATMENT OF ZINCAND METALLIC MATERIALS COATED WITH ZINC Ryuichi Sano, Nishinomiya-shi,and Zenji Ota, Amagasaki-shi, Japan, assiguors to Kansai Paint Company,Limited, Amagasaki-shi, Japan No Drawing. Filed Dec. 6, 1965, Ser. No.511,979 Claims priority, application Japan, Dec. 7, 1964, 39/ 68,448Int. Cl. B01k 5/02; C23c 13/02 US. Cl. 204-181 17 Claims ABSTRACT OF THEDISCLOSURE This invention relates to a new anticorrosive treatment ofzinc and metallic materials coated with Zinc. Its characteristic is toform a complex anticorrosive film consisting of a resin-chrome-zinccompound and zinc chromate on the surface of the above metals by bakingafter passing direct current through an aqueous solution composed ofWater-soluble or water-dispersible resinates and chromates orbichromates or both of these salts using zinc (including zinc alloy) ormetallic materials coated with zinc (including zinc alloy) as anode.

The resinates used in this invention are ammonium (or amine) salts ofcomparatively large amount of carboxyl group-containing alkyd resins,acrylic acid resins, phenolic resins, carboxylic acid adduct drying oilsand epoxy ester resins, which are soluble in water, forming resinateion, or are dispersible in water into the colloidal state.

The alkyd resins are the per se conventional esters of dibasic acid withglycerol or polyglycol. The acid may be phthalic acid (phthalic acidtype alkyd resins), trimellitic acid (trimellitic acid type alkydresins), pyromellitic acid (pyromel'litic acid type alkyd resins), etc.Moreover, these alkyd resins can be modified in per se conventionalmanner.

In the present invention, for example, as alkyd resinates, use can bemade of ammonium salts or organic amine salts of phthalic acid typealkyd resins, trimellitic acid type alkyl resins, pyromellitic acid typealkyd resins, fatty acid-maleic anhydride adduct modified alkyl resins,fatty acid-furnaric acid adduct modified alkyd resins, dimethylolpropionic acid type alkyd resins, rosin-maleic anhydride and other alkydresins having an acid value greater than 30.

The acrylic acid resins comprise the per se conventional polymers ofacrylic, substituted acrylic and methacryic acilds, their salts, esters,and other derivatives, such as nitriles and amides.

As acrylic system resinates, use can be made in the present invention offor example ammonium salts or organic amine salts of carboxylgroup-containing acrylic system resins with an acid value above 30,obtained by copolymerization of the following materials: (1) acidiccopolymer of one or more of acrylic system monomers such as alkylacrylate, (e.g. methyl acrylate, etc.), alkyl methacrylate, (e.g. methylmethacrylate, etc.), aryl acryl- Patented Apr. 8, 1969 ate, (e.g. phenylacrylate, etc.), aryl methacrylate, (e.g. phenyl methacrylate, etc.),glycidyll acrylate, glycidyl methacrylate, acrylonitrile and acrylamidewith unsaturated carboxylic acids such as acrylic acid, maleic acid andfumaric acid and (2) the above mentioned acrylic system acidiccopolymers modified with vinyl system monomers such as styrene, vinyltoluene, vinylidene chloride, vinyl chloride and vinyl acetate.

The phenol or phenolic resins comprise the products made by reacting aphenol (phenol, cresols, xylenols, p-t-butyl-phenol, p-phenylphenol,bisphenols, resorcinol, etc.) and an aldehyde (formaldehyde, furfural,etc.), as well as the conventionally modified phenolics.

As phenol resinates, use can here be made of ammonium salts or organicamine salts of carboxylic acid group-containing phenolic resins of anacid value over 30, such as phenolic resin modified with diphenolic acidand phenolic resin modified with salicylic acid.

As salts of carboxylic acid adduct drying oil, there are employedammonium salts of organic amine salts of carboxylic cid aadduct dryingoil having an acid value over 30, such as, for example, additioncompounds of polybasic acid having an a,/3-ethylenic linkage flanked bycarbonyl or carboxyl groups, such as maleic anhydride, fumaric acid withvegetable oil, animal oil and synthetic oil.

Salts of epoxy ester are for example ammonium salts or organic aminesalts of epoxy ester modified with carboxylic acid adduct drying oilhaving an acid value over 30 and epoxy ester modified with polybasicacid.

In the above cases, resins can be used which dissolve or emulsify inwater due to the presence of hydrophilic radical besides carboxyl group.

Chromates and bichromates to be used in this invention comprisewater-soluble chromates and bichromates such as sodium chromate,potassium chromate, ammonium chromate, sodium bichromate, potassiumbichromate and ammonium bichromate. The aqueous solution and waterdispersed composition to be used in this invention are the aqueoussolution and aqueous dispersion, respectively, having a pH over 7obtained by adding chromate or bichromate to the above-describedresinates. In this case, the aqueous solution of bichromate such assodium bichromate, potassium bichromate and ammonium bichromate isitself acidic (pH below 7); however, in the compositions of pH over 7used in this invention, bichromate ion is observed to be converted tochromate ion.

Therefore in the aqueous solution of water-dispersed composition used inthis invention, the water-soluble bichromates have the same effect asthe water-soluble chromates. The aqueous solution and water dispersedcomposition to be used in this invention require a pH over 7, and asuitable range is from pH 7.2 to pH 8.8.

If the pH becomes acidic, the resinates containing ammonium salt ofcarboxylic acid or organic amine salt of carboxylic acid as hydrophilicradical generally become unstable in the aqueous com-position, and theirstorage stability is decreased; therefore operation of the method ofthis invention becomes diflicult.

In alkaline solution, when a Zinc material is immersed as anode in anelectrolyte, and direct current is applied for a short period, a yellowfilm is easily formed. The yellowish material formed on zinc anode is aresinatecontaining mixture (resin acid and zinc resinate, chromate ionand zinc chromate). Zinc chromate thus formed on the surface of zincmaterials is confirmed by X-ray diffractometry to be the same as Z.T.O.type zinc chromate (Zinc tetroxychromate, ZnCrO -4Zn(OH) is referred toas Z.T.O. hereinafter). On the contrary, in acidic range solution, zincchromate does not form and consequently the anticorrosive property ofthis invention is completely lost.

It is generally recognized that chromate ion and bichromate ion are inequilibrium in an equeous solution, and bichromate ion is predominantlyformed in acidic range and chromate ion in alkali range. Zinc bichromatewhich is considered to be formed when electric current is applied byimmersing zinc material to be coated in an equeous treating solution ofacidic range, has a very high solubility in water compared with zincchromate which is formed in alkaline solution; therefore the fact thatno stable chromate treated layer is formed in acidic solution can beclearly confirmed by tests.

Z.T.O. type zinc chromate is a very effective material for corrosionprevention, i.e. as rust inhibitive pigment, and it is the importantfactor in ensuring the splendid corrosion resistance of the film formedby the method of this invention. For the formation of zinc chromate, theexistence of metallic zinc material is necessary.

The concentration of chromate ion in the composition should be over 0.1part in 100 parts of the composition by Weight, and suitably should beover 0.5 part against 100 parts of resinate in the composition.Non-volatile matter of the composition is suitably 5(i% (by weight).

Organic solvent may be added to the composition in order to control thedispersed state of each composition, as well as viscosity and dielectricproperty. Also pigment, dye and other tinting agent may be added to thecomposition to enhance the appearance effect by coloring the chemicalfilm.

In carrying out the method of this invention the composition of thesesolutions is desirably kept constant as much as possible during currentapplication; therefore when a composition change becomes detrimental forthe operation of this invention, lacking ingredients are added orexcessive elements are removed by vaporization or other means to adjustthe composition.

For removing excessive ingredients such as alkali, water and organicsolvents of the composition of this invention, or for preventing thereaction of chromate on the cathode surface, the use of diaphragms suchas porous porcelain plate, ion exchange membrane, asbestos membrane,nylon membrane and the like having selective ion permeability andelectro-osmotic property, are effective.

metallic plate coated with zinc metallizing or with zincrich paint; alsoordinary zinc or zinc alloy. The metallic zinc materials may be in anyshape, such as plates, castings, etc.

The electric current application of this invention is carried out byimmersing the metallic zinc material to be treated in the above aqueoussolution or in the water dispersed composition and using it as anode.Another suitable electrode, for example, iron, aluminum, platinum, orcarbon is used as cathode, and the current is passed betwen theelectrodes. Since the amount of deposition of the chemical film is ingeneral proportional to the quantity of electricity passed through theelectrodes, the necessary film thickness can be controlled by adjustingthe quantity of electricity.

The quantity of electricity can be easily adjusted by changing voltagebetwen the two electrodes or by changing the electricity-passing time.If the voltage between the electrodes is kept constant, the largestcurrent flow takes place at the beginning of current application, andthe current decreases as the electric resistance of the film formed onthe anode surface increases.

After a long period of time, the current approaches zero. For a similarreason, voltage between the electrodes must be increased with time inorder to keep the current constant. In order to obtain an effectivechemical film in the method of this invention, voltage between theelectrodes is suitably above 3 v., and the current density of the anodesurface should be kept above 5 ma./dm.

The first characteristic of the method of this invention is the obtainedvery superior anticorrosive property, as is hereinafter demonstrated.

Various anticorrosive treatments including the method of this inventionare applied to ordinary zinc galvanized steel sheet, and theirrespective anticorrosive properties are compared in terms of the timerequired to form white rust on the metal surface by salt-spray test.Japanese Industrial Standard Z 237i salt-spray test method whichcorresponds to A.S.T.M. Designation: B 117-61 Method of Salt-spray (Fog)Testing which has a close relation with anticorrosive property ofmetals.

The results are shown in the following table, and it is evident that theeffect of the anticorrosive treatment of this invention is far betterthan that of the usual chemical treatments.

TABLE.COMPARISON OF PROPERTIES BETWEEN VARIOUS ANTICOR- ROSIVETREATMENTS ON ZINC GALVANIZED STEEL SHEET Time required Thickness torforma- No. Method of treatment of treat tion of Remarks merit; filmwhite rust (micron) in the saltspray test (hour) 1 No treatment 0 1 2Chemical treatment with 0. 5 48 chromic acid, sulphuric 3 acid), sodiumsulphate. 4 O omparispn to 4 Zinc phosphate chemical 1 24 g i treatmentafter pickling t with hydrochloric acid. rea men Etching primer 5 60 BThe method of this invention 2 7. do 4 200 8. Thermosctting acrylicresin 25 Comparison to coating. conventional 9... The method of thisinvention- 25 800 paint him. 10 Zinc phosphate chemical 1 150 treatmentafter pickling with hydrochloric acid.

plus Composition 1'01- Thennosetting acrylic resin 15 chemicaltreatcoating. merit plus 11 The rlnethod of this invention- 5 1, 000paint film.

p us Thermosetting acrylic resin 10 coating.

Metallic zinc materials suitable for this invention include all metallicmaterials coated with film or particles of metallic zinc (including zincalloy), which conduct electricity through the metallic zinc coating,such as elec- The reason that the anticorrosive property of the metalliczinc materials treated by the method of this invention is far superiorto those of the usual methods is considered to be as follows: in themethod of this invention, Z.T.O.

trically galvanized sheet, hot dip galvanized sheet, and 75 type zincchromate chemical film is uniformly and densely formed on zinc byelectrolysis reaction through direct current application in aqueousalkaline solution, and it forms a kind of resistance film; at the sametime molecular or colloidal particles of the resin, which are negativelycharged by dissociation of the carboxylic salt, deposit on the anodesurface by electrophoresis, electro-osmosis and anodic electrolysis, andpartly react with zinc ions which are dissolved from the anode to formwater-insoluble zinc resinate, the formation of which is observed byinfrared spectroscopic analysis; on the other hand, chromate ion iselectrically accumulated inside the film.

If the film consisting of the above compositions is cured by heating,very complex anticorrosive film is formed combining substrate zinc,Z.T.O. type zinc chromate, zinc resinate and resin by chemical andphysical bond formation, in addition to the coordinate bond between thepolar radical in the resinate and Cr formed by the reduction of chromicacid ion.

Thus, a film of a very splendid anticorrosive property is obtained as isshown in the preceding table.

The second characteristic of this invention is a very good adhesion ofthe chemical film to the metallic zinc material. This is due to the factthat the chemical film of this invention is chemically and physicallybonded to zinc.

A chemical film of below 5 microns thus deposited on zine galvanizedsteel sheet does not peel or check in various punching or formingoperations, or in an ordinary bonding test and scratch hardness test.

The third characteristic of this invention is that the film can becolored freely. By using pigment, dye or other tinting agent in thecomposition solution of the invention, anticorrosive treatment andcoloring of metallic zinc material are simultaneously carried out.Therefore without a finish ing coat, sufi'icient anticorrosive effectand enhanced appearance effect are concurrently obtained.

The fourth characteristic of this invention is that the surface of thefilm is smooth, and that uniform film can be formed even in the narrowspace of the treating metal, due to large electrical resistance of zincchromate. When zinc chromate is formed on the anode surface at theinitial stage of the current application, it forms a dense protectivefilm and increases the resistance between the electrodes; thus, whenchromate ions are accumulated in the film formed, and even if theconcentration is increased, nonuniformity of current density over theanode surface is essentially excluded, and therefore a smooth surface iseasily obtained. The formation of protective film by zinc chromate canalso be seen from the phenomenon that the resistance increase betweenthe electrodes is greater for the case that water-soluble chromateattains a certain concentration than for the case wherein the current ispassed through the anode of zinc material in an aqueous solution ofwater-soluble resinate only. If a steel panel is used as anode insteadof zinc material, resistance between the electrodes decreases uponaddition of a small amount of water-soluble chromate, and markedcrawling is observed in the film formed. The reason is that theprotective film is not formed on the surface of steel sheet byinterfacial electrochemical reaction of chromic acid ion, such as isobserved in the case of metallic zinc material. Therefore, as good ananticorrosive effect for general steel materials can not be realized bythe method of this invention as for zinc materials.

The fifth characteristic of this invention is that the composition hasgood storage stability. Generally if a large amount of water-solublechromate or bich-romate is added, the storage stability of the compoundtends to become inferior. Also if chromate system protective pigmentssuch as zinc chromate and strontium chromate are added in large enoughamount to obtain sufficient anticorrosive property, storage stability isimpaired. In the method of this invention, since chromate ionconcentration in the film formed on the anode surface by currentapplication is increased to a value higher than the concentration in theaqueous composition, even when storage stability is increased bylowering the chromate ion concentration in the aqueous composition, theobtained anticorrosive property is superior.

The sixth characteristic of the invention is that a good ground coat isobtained by a very short time treatment. If the electric currentapplication of 10 seconds under the interpolar voltage of 20-50 v. isused, a film of 3 to 12 microns is obtained, and if the current ispassed 1 second under interpolar voltage of v., a film of 2 microns isobtained after baking, and when conventional coatings are overcoated onthe film, its anticorrosive property and adhesive property are markedlyincreased, and the film shows far better properties than can be achievedby the chemical treatment of the usual zinc phosphate system and chromicacid system.

The seventh characteristic of this invention is that the adhesiveproperty of the coatings is very good. The film formed by this inventionhas a good adhesion not only to the metallic zinc substrate, since theresinate used in this invention has large polarity, but also toovercoating film such as alkyd type, aminoalkyd type, acrylic type,phenol type, epoxy type, urethane type, oil type and other generalcoatings.

Presently preferred illustrative embodiments of the invention are setforth, solely by way of example, in the following, wherein percentagesare by weight:

EXAMPLE 1 60 parts by weight of butyl alcohol is added to parts byweight of trimellitic acid-alkyd resin (modified with linseed oil fattyacid; acid value 60) whereupon dissolution takes place; 6.5 parts byweight of 28% ammonia water is added to the solution, which is agitatedthoroughly for neutralization. 500 parts by weight of water is furtheradded, thereby producing water-soluble varnish. To this 35 parts byweight of 15% ammonium chromate aqueous solution is added to make up atreatment composition, the pH of which is 8.1.

The thus-prepared composition is placed in a treatment tank. Afterdirect current is passed through the said composition for 2 minutesunder the interpolar voltage of 30 v. using a zinc galvanized steelpanel as anode and a steel panel as cathode, the zinc galvanized steelpanel is taken out from the tank, and any water-soluble component,adhering to the surface of the film formed, is washed out by flowingwater or by water spray, and then the treated film is cured by bakingfor 30 minutes at C. The film formed on the zinc galvanized steel panelis of 26 micron thickness. No white rust or the like is observed to beformed at all on the thus-treated panel for over 900 hours as a resultof the salt-spray test, the panel thus showing a very good anticorrosiveproperty.

EXAMPLE 2 20 parts by weight of butyl Cellosolve is added to 100 partsby weight of trimellitic acid-alkyl resin (modified with dehydratedcastor oil fatty acid; acid value 80), whereupon dissolution take place.To the resultant solution 12.7 parts by weight of dimethylaminoethanolis added, and the Whole agitated well for neutralization. 530 parts byweight of water is further added, thereby producing water-solublevarnish. 2 parts by weight of strontium chromate is added to the latter,and dispersion carried out for 24 hours in a ball mill. Afterdispersion, 15 parts by weight of 25% ammonium chromate aqueous solutionis added, a treatment com-position being thus obtained. The pH of thistreatment composition is 8.5.

The so-prepared composition treatment is placed in a tank. After directcurrent is passed for 30 seconds through the composition, keeping theinterpolar voltage at 50 v., and using zinc galvanized steel panel asanode and an aluminum plate as cathode, the zinc galvanized steel panelis taken out from the tank, and water-soluble component adhering to thesurface of the film formed is washed out by water fiow or by waterspray, and the panel then baked at C. for 15 minutes. A film of 27micron thickness 7 is formed on the zinc galvanized steel panel. Theformation of white rust or the like is not observed at all on the panelfor over 1000 hours in the salt-spray test, the panel thus showing avery good anticorrosive property.

EXAMPLE 3 20 parts by weight of butyl alcohol is added to 100 parts byweight of fumaric acid-tung oil adduct (acid value 150), with resultantdissolution. To the thus-obtained solution, 12 parts by weight ofdimethylamine is added for neutralization. Then 500 parts by weight ofwater is added to make up a watersoluble varnish. After adding 10 partsby weight of phthalocyanine blue, the mixture is subjected to dispersionfor 24 hours in a ball mill. After dispersion, 6 parts by weight of 20%potassium chromate aqueous solution and 4 parts by weight of 20%potassium bichromate aqueous solution are added to obtain a treatingcomposition, the pH of which is 7.8. This treating composition is putinto a treating tank.

After direct current is passed through the treating composition for 5minutes, keeping the interpolar voltage at 20 v. and using a zincmetallizing steel plate as anode and a steel panel as cathode, the zincmetallizing steel panel is taken out, and water soluble component on thesurface of the film formed is Washed out by water flow or by waterspray, after which the panel is baked at 200 C. for minutes. A tintedfilm of 30 micron film thickness is thus formed on the zinc metallizingsteel panel. No white rust or the like is observed on this panel forover 1000 hours in the saltspray test, the panel thus showing a verygood anticorrosive property.

EXAMPLE 4 40 parts by weight of ethyl Cellosolve is added to 100 partsby weight of maleic acid-linseed oil adduct (acid value 100), withresultant dissolution. 10.8 parts by weight of 28% ammonia water isadded further for neutralization. By adding 517 parts by weight ofwater, a water-soluble maleic acid-linseed oil adduct varnish ofnon-volatile matter is made up. Next 6 parts by weight of ethylCellosolve is added to 15 parts by weight of phenol resin modified withsalicylic acid (acid value 80) with resultant dissolution; further 1.3parts by weight of 28% ammonia water is added for neutralization. Then78 parts by weight of water is added to make up a water-soluble phenolresin varnish of 15% non-volatile matter. The water-soluble maleicacidlinseed oil adduct varnish is mixed with the water-soluble phenolresin varnish, and further 33 parts by weight of 15% ammonium chromateaqueous solution is added to make up a treatment composition, the pH ofwhich is 8.3. This composition is put into a treatment tank.

Direct current is passed through the composition for 10 seconds, at 30v., using a zinc galvanized steel panel as anode and a steel panel ascathode. Then excessive composition adhering to the surface of the filmformed is squeezed out by a gum roller, and the coated zinc galvanizedsteel panel is baked at 300 C. for 1 minute. A film of 4 micronthickness is formed on the zinc galvanized steel panel, and no whiterust or the like is observed to be formed thereon for over 200 hours inthe salt-spray test. It shows a very superior anticorrosive propertyrelative to ordinary zinc galvanized steel panel chemically treated.Aminoalkyd resin coating of microns thickness is coated on the processedzinc galvanized steel panel by his treatment; this panel shows no whiterust or the like at all after over 900 hours of the salt-spray test,thus exhibiting a very good anticorrosive property.

EXAMPLE 5 To 200 parts by weight of a water-soluble acrylic resin (acopolymerized resin of 3 moles of ethyl acrylate, 3 moles of butylacrylate and 4 moles of ammonium acrylate; having 50% of non-volatilematter; solvent being a mixture of water and ethanol in the weight ratioof l to l), 25 parts by weight of 20% sodium chromate aqueous solutionand 480 parts by weight of water are added to make up a treatingcompound of pH 7.5.

Electric current is applied to a zinc galvanized steel panel accordingto the method described in the preceding Example 4, and the resultantcoated panel is subjected to forced drying. A film of 5 micron thicknessis thus formed On the zinc galvanized steel panel, and no white rust andthe like is observed for over 220 hours in the salt-spray test. It showsa very superior anticorrosive property compared to ordinary chemicaltreated zinc galvanized steel panel. When a thermosetting acrylic resincoating of 10 micron thickness is coated on the zinc galvanized steelpanel treated by this method, and is tested by the salt-spray test, nowhite rust and the like is observed for over 1000 hours, thus showing avery good anticorrosive property.

EXAMPLE 6 To 200 parts by weight of a water-soluble acrylic resin (acopolymerization product of ethyl acrylate 3 moles, butyl acrylate 3moles and ammonium acrylate 3 moles; having 50% of non-volatile matter;the solvent being a mixture of water and ethanol in the weight ratio of1 to 1), 5 parts by weight of carbon black and 755 parts by weight ofwater are added, and the carbon black is then dispersed uniformly in asand mill. Then parts by weight of acrylic emulsion (commerciallyavailable as Rhoplex AC 200 having 46% of non-volatile matter) and 35parts by weight of 20% ammonium chromate solution are added to make up atreating composition of pH 7.8.

This is charged into one side of a treating tank which has a biscuitfired diaphragm, and to the other side 0.5% ammonia water is charged.Direct current is passed for 20 seconds under 200 v. using a zincgalvanized steel panel placed in the composition side as anode, and asteel panel placed in the ammonia water side as cathode. Afterwater-soluble component adhering to the surface of the film formed iswashed out by water flow or by water spray, forced drying is carried outat 350 C. for 30 seconds. A tinted film of 6 microns is formed on thezinc galvanized steel panel, which shows no white rust or the like forover 250 hours in the salt-spray test.

The decrease of gloss after 1000 hours of weatherometer irradiation isbelow 20%, and the surface condition is kept good. Scratch hardnesstest, impact test, Erichsen test and other physical tests show almostthe same values as those before the irradiation, and are very good.

What is claimed is:

1. A method of rendering a' zinc-containing substrate resistant tocorrosion, which comprises forming an anticorrosive film on the surfaceof the said substrate by (a) immersing the substrate as anode in anaqueous electrolyte at a pH above 7, said electrolyte containing (1)ammonium or organic amine resinate containing COO group as hydrophilicradical and (2) a member selected from the group consisting ofwater-soluble chromate, water-soluble bichromate and mixtures thereof,(b) passing direct current through said electrolyte from cathode toanode whereby the objective anticorrosive film is deposited on thelatter, (c) discontinuing the current passage when the desired thicknessof film has been thus deposited, and (d) withdrawing the coated anodefrom the electrolyte and subjecting the withdrawn coated anode tothermal curing.

2. A method according to claim 1, wherein the sub strate is zinc metal.

3. A method according to claim 1, wherein the substrate is zinc alloy.

4. A method according to claim 1, wherein the substrate is zincgalvanized sheet steel.

5. A method according to claim 1, wherein the said pH is in the rangefrom 7.2 to 8.8.

6. A method according to claim 1, wherein the deposited film compriseszinc chromate.

7. A method according to claim 1, wherein the deposited film comprisesWater-soluble ammonium or organic amine resinate.

8. A method according to claim 1, wherein the deposited film compriseszinc chromate and water-soluble ammonium or organic amine resinate.

9. A method according to claim 1, wherein the elec trolyte contains morethan 0.1 part by weight of chromate ion per 100 parts by weight ofelectrolyte.

10. A method according to claim 1, wherein the electrolyte contains morethan 0.5 part by weight of chromate ion per 100 parts by weight ofresinate.

11. A method according to claim 1, wherein the voltage is above 3 voltsand the current density at the anode is greater than 5 ma./dm.

12. A method according to claim 1, wherein the resin is trimellitic acidtype alkyd resin.

13. A method according to claim '1, wherein the resin is fumaricacid-tung oil adduct.

14. A method according to claim 1, wherein the resin is maleicacid-linseed oil adduct.

15. A method according to claim 1, wherein the resin is copolymer ofethyl acrylate, butyl acrylate and ammonium acrylate.

16. A method according to claim 1, wherein the deposited film isovercoated with a resin film.

'17. A method according to claim 16, wherein the resin is acrylic resin.

References Cited UNITED STATES PATENTS 2,412,543 12/1946 Tanner 204-38422,469,015 5/ 1949 Stareck et al. 204-56 2,851,386 9/1958 Hartman 14 86.23,011,958 12/1961 White 204--56 3,036,934 5/1962 Horton et al. 1486.23,053,692 9/1962 Pocock 148-6.2 3,132,055 5/1964 Tanaka et al. 1486.163,175,964 3/1965 Watanabe et a1 204-481 3,330,744 7/1967 Wright et a120456 3,378,477 4/1968 Gentles et al 204-181 HOWARD S. WILLIAMS, PrimaryExaminer. E. ZAGARELLA, JR., Assistant Examiner.

US. Cl. X.R.

